Electrical impedance occurs when there is opposition between a source that transmits an electric pulse and a receiver of that electric pulse. Impedance is a combination of two components, resistance and reactance. Resistance is generally in phase with the source signal and is represented by the real component of the impedance. Reactance refers to the out-of-phase portion of the impedance and is represented by the imaginary component of the impedance. When the impedance of a driver and that of its load, which may include a transmission line and/or a receiver, are not matched, then signals transmitted from the driver will be reflected by the load. These reflections cause noise and ringing in the signal. In an arrangement with a driver, a receiver and a transmission line, impedance mismatch may occur between the driver and the receiver as well as between the driver and the transmission line. Thus, there may be two reflections of a transmitted signal; one from the driver's mismatch with the receiver, and one from the driver's mismatch with the transmission line. Reflections from impedance mismatch should be avoided in circuit design.
The impedance of the driver can be matched to the impedance of the transmission line by adjusting the voltage-to-current (voltage/current) characteristics of the driver. Efforts to accomplish this impedance matching are tedious and may not be accurate because of the introduction of parasitics into the driver circuit during adjustments. Furthermore, the output impedance of a driver is typically adjusted once and then set. The driver output impedance, then, is insensitive to changes in the impedance of the transmission line.
There is a need, therefore, to provide more effective ways of correcting for impedance mismatch in the transmission of a pulse from a driver to a load.